Numerous applications require controlled motoring and braking of an electric machine. In automotive and other applications powered by a rechargeable DC source, such as a storage battery, the control requires at least a pair of switching devices connected across the DC bus for supplying regulated current to the armature windings of the machine during motoring, and at least a pair of associated rectifiers for returning generated current to the battery during braking. When the generated voltage is less than the battery voltage, the switching devices may be modulated to aid the induced voltage in the manner of a boost converter. Typical automotive applications include propulsion and active suspension; see, for example, Murty et al. U.S. Pat. No. 5,091,679, issued on Feb. 25, 1992, and assigned to the assignee of the present invention.
Under certain conditions, such as when the battery is fully charged and/or when the motor speed is such that the induced voltage (back-EMF) exceeds the DC bus voltage, the braking force cannot be conveniently controlled. In the latter case, the rectifiers conduct almost continuously since the DC bus voltage is limited substantially to the terminal voltage of the battery. In the case of the fully charged battery, the generated current must be supplied to an alternate load, such as a bank of load resistors connected in parallel with the battery, as in the above-referenced U.S. Pat. No. 5,091,679.
In applications where controlled braking is essential, it is known to regulate the generated current by modulating the conduction of a resistive load circuit connected across the DC bus. See, for example, the braking control disclosed in Murty et al. U.S. Pat. No. 4,815,575, issued on Mar. 28, 1989, and assigned to the assignee of the present invention. In an application requiring both motoring and braking, however, this approach requires separate current controllers for motoring and braking, and circuitry for controlling transitions between the two modes of current control.